Rotary airlock valves are well known for use in the solids handling industry. Often, a solid material is transported to or received in a vessel using a pressurized gas such as air. The solid material accumulates in the vessel which remains under pressure. The typical way to remove the solid material is for it to collect in a discharge area or hopper which leads to an entrance of the rotary airlock valve.
A rotary airlock valve typically comprises a housing containing a cylindrical chamber within which is mounted an axially extending shaft. The shaft has a plurality of radially extending vanes or blade portions, with material receiving pockets defined between the vanes. As the shaft rotates, the outer edges of the vanes sealingly engage the inner surfaces of the cylindrical chamber wall. The rotary airlock valve typically has an upper inlet opening for receiving material, which falls into the receiving pockets between the vanes, as the shaft is rotated. There is also an outlet opening, typically on the bottom thereof, which allows the material to fall out of the material pockets, as each successively rotates into a discharge position. Between the inlet and outlet openings, the vanes defining the material receiving pockets are in sealing engagement with the chamber walls, to prevent any by-pass of pressurized gas from the inlet opening to the discharge opening. The rotary airlock valve thus prevents a direct connection between the inlet side, which may be under high pressure, and the outlet side which may be at a lower or atmospheric pressure. Thus, the airlock valve enables transfer of solid material from the pressurized vessel, to a conveyor or other vessel at a lower pressure, at pressure differences possibly on the order of 5 to 50 mbars.
In some applications, such as when drying a solid material, such as finely divided biological materials i.e., wood pulp, sugar beet pulp or sugar cane pulp, steam may be used with the product to be transferred. For example, in the manufacture of paper pulp, wood chips are fed to a hopper at up to 12 bars pressure, the chips then fed through the airlock valve to a digester. FIGS. 1a and 1b are views of such a rotary airlock valve. The rotary air lock in that case is typically designed to have a rotor having a minimum number of large material receiving pockets, with a large sealing surface in between the pockets, owing to the high pressure differential. These airlock valves also may be provided with adjustable cylinder walls that can be moved to improve the seal between the rotor vanes and the cylinder walls.
However, there are problems with rotary airlock valves in such an application, particularly due to wear. Whenever solids transfer occurs, there is a potential for the sealing surfaces to wear, resulting in gas by-pass around the rotary air valve. This is a particularly serious problem when handling pulp, as this may contain sand, which is quite abrasive. Also, the rotary airlock valve typically includes a seal or gasket between the rotating vanes and the static housing portions, and any solids incursion can lead to rapid wear, that can result in gas leaks outside of the housing. When such leakage occurs, the system must be shut down, resulting in costly processing delays. Typically, such rotary airlock valves have a very limited life in the harsh pulp service described above, of about two months.
This problem is particularly acute when the rotary airlock valve transfers a dried solid material from a pressure of about 2.5 bar, to atmospheric pressure, with superheated steam being the pressurizing gas. As the shaft rotates to the discharge opening, the initial breach to the low pressure side causes a strong outflow of steam, causing the dried material be accelerated as it is driven out of the pocket, at velocities of up to 300 meters per second, increasing the abrasive effect and wear on the adjacent parts, particularly the edge of the opening. In addition, the flashing of the steam from the pocket can cause condensation and wetting of the powder material or adjacent surfaces, that may lead to product accumulation.
With each successive turn of the room, the expansion effect increases due to a wearing of the sealing surfaces, eventually resulting in a gas by-pass around the valve.